narrowing the GDK include guard to #if defined(HAVE_GTK3), and
removing legacy HAVE_GTK2 / HAVE_GTK toolkit label branches from sysinfo_detect_toolkit(),
narrowing GDK display probing guard in sysinfo_detect_display_backend() to GTK3-only.
Updated Windows and macOS sysinfo backends to remove HAVE_GTK2 / HAVE_GTK fallback labels so sysinfo_detect_toolkit() now returns only "GTK3" (or NULL).
Updated API docs for sysinfo_backend_get_ui() to remove the outdated "GTK2 / X11" example so comments match runtime behavior.
Removed the deprecated Meson option option('gtk3', ... deprecated: true) from meson_options.txt, leaving gtk-frontend/text-frontend options intact.
Removed -Dgtk3=true from Debian’s Meson configure flags in debian/rules (override_dh_auto_configure).
Removed remaining active -Dgtk3=true packaging consumers in Flatpak config:
dropped it from config-opts
dropped -Dgtk3=true from cflags.
Removed -Dgtk3=true from Solus packaging Meson configure args.
Committed changes on the current branch (e018bfd) and created a PR record via the make_pr tool.
